Transcript Study of Belle Silicon Vertex Detector Intrinsic Resolution

Various Work for Belle Detector
FGIP-student Forum
TIT, 2005-06-17
Saša Fratina,
Jožef Stefan Institute, Ljubljana, Slovenia
Outline of the talk
A little bit about Slovenia.
Ring Imaging Cherenkov
Counter (RICH)
 Silicon Vertex Detector (SVD)
 CP-asymmetry measurement
in B meson system

Where do I come from?
Slovenia
Austria
Italy
Hungary
Croatia
Few facts： 2M inhabitants, 20.000 km2, capital
Ljubljana, EU members (www.slovenia.info)
Education system: public schools
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
Primary school (9 years, children 6-15 years old)
Secondary school – high school (4 years,
students 15-19 years old), at the end of which
you have to pass national exams from 5 subjects
(slovene language, math, english and two by
student`s choice)
University (4-5 years for graduation, usually
takes longer)
Graduate studies: masters or PhD course
depending on the field, in physics mostly PhD.
Universities in Slovenia
University of Ljubljana (largest,
22 faculties, 50.000 students)
 University of Maribor
 Nova Gorica Polytechnic

My education



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Primary school: Ljubljana
High school: Gimnazija Bezigrad,
Ljubljana (finished with International
Baccalaureate)
graduated at the University of
Ljubljana, Faculty of Mathematics
and Physics
started PhD study in 2002 at the
University of Ljubljana (and working
at Jozef Stefan Institute, Ljubljana)
About work…
Belle Control room
Main goal of Belle experiment:
study of CP violation in B-meson system
Mt. Tsukuba
KEKB
Belle
B
e+
B
 z ~ 200 m
U(4s)
U(4s)
p(e+)=
3.5 GeV/c
ep(e-)=
8.0 GeV/c
Ring
Imaging
Cherenkov
Counter
Silicon vertex detector
Data
analysis:
study of CP
violation
RICH for super-Belle (2008?)
Requirements
 Compact detector
 Good /K separation in the forward (high momentum) region
 for few-body decays of B's (B   , B   K)
 for b -> d g , b -> s g (   ,   K  )
 Low momentum (<1GeV/c) e/μ/ separation (B ->Kll)
 High efficiency for tagging kaons
Basic principle

Ring Imaging Cherenkov counter, RICH
cos Ch = 1/n
 = v/c
aerogel
particle
Cherenkov photons
Position
sensitive
photon
detector
Aerogel




 Ch () ~ 308 mrad
 Ch () - Ch (K) ~ 23 mrad
few cm thickness
100x100x20mm n=1.050
transmission length
2.5 - 4.5 cm
approx. 10 emitted
photons / cm
n = 1.05
3
No cracks
Photon detector
photo multiplier tube
(PMT)
 single photon sensitive
 position sensitive:



position resolution few
mm
quantum efficiency 
20%
Photon detector: array of
16 H8500 PMTs
Beam test measurements
 Confirm feasibility of
such detector
 Study /K separation
capability
Clear rings, little background
Beam test: Cherenkov angle
resolution and number of photons
Beam test results with 2cm thick aerogel tiles:
>4s K/ separation
Typically around
13 mrad (for 2cm
thick aerogel)
-> Number of photons has to be increased.
How to increase the number of photons?
What is the optimal radiator thickness?
Use beam test data on s0 and Npe
s0
s=s0/(Npe)
Npe
Minimize the error per track:
s=s0/ (Npe)
Optimum is close to 2 cm
Radiator with multiple
refractive indices
How to increase the number of photons
without degrading the resolution?
measure two separate rings
“defocusing” configuration
●
normal
measure overlaping rings
“focusing” configuration
●
FOCUSING CONFIGURATION - data
4cm aerogel single index
2+2cm aerogel
Increase the number of photons without degrading the resolution!
FOCUSING CONFIGURATION
– momentum scan
single photon resolution:
dual radiator ~same as single (of
half the thickness) for the full
momentum range
●
number of detected hits: dual
radiator has a clear advantage
●
Development and testing of
photon detectors for 1.5 T


Baseline: large area HPD of the proximity focusing type
Backup: MCP-PMT (micro channel plate)
Multialkali
photocathode
-10kV
15~25mm
e-
Pixel PD or APD
R&D project in collaboration with HPK
RICH - conclusions
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Feasibility of RICH detector was confirmed.
More photons: employ radiators with multiple
refractive indices. Idea successfully tested in
beam tests.
Aerogel production: transmission length
improved, new cutting methods tested, multiple
layer samples.
R&D issues: development and testing of a
multichannel photon detector for high mag. fields
20 cm
50 cm
Silicon vertex detector
basic SVD unit:
Double Sided Strip Detector (DSSD)
separate
measurement of r
and z coordinate
pitch
strip pitch:
50 and 75 m for
r and z coordinate,
respectively
Evaluate the performance of SVD
– measure its intrinsic resolution:
Incident angle dependence, occupancy study,
alignment cross-check
SVD intrinsic resolution
Error on the track position
measurement
 Track position
residual
is determined
from the SVD
SVD hit
hits on other layers

track
DSSD
Typical residual distributions
r coordinate
z coordinate
residual [cm]

residual [cm]
Intrinsic resolution is determined from the width
of the residual distribution:


si = 10 m, RMS = 12-15 m for r and
si = 25 m, RMS = 30 m for z coordinate
Incident angle dependence
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
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Simple estimate for the perpendicular tracks:
signal collected by single strip → resolution ≈
strip pitch / 12
Small incident angle: signal collected by few
y
strips → resolution improved
Large incident angle: signal
track
collected by many strips
→ resolution gets
worse due to smaller
x
signal to noise ratio
x strips
Incident angle dependence: result
r coordinate
z coordinate
RMS [m]
60
RMS [m]
30
20
40
10
20
0
0
-20
0
20
40
Incident angle [degrees]
Innermost layer
-20
0
20
40
60
Incident angle [degrees]
Incident angle dependence: result
r coordinate
z coordinate
RMS [m]
60
RMS [m]
30
20
40
10
20
0
0
-20
0
20
40
Incident angle [degrees]
-20
0
20
40
60
Incident angle [degrees]
Different colors show the result for all four layers:
black, red, green and blue for the innermost, second, third and outermost layer.
Magnetic Field Effect
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Intrinsic resolution is not symmetric with respect to
perpendicular
incident angle
Distribution of hits
Reason:
with cluster size
magnetic field,
1 strip, 2 strips, 3 strips, …
confirmed
by the plot of
hit cluster size
next SVD:
-20
-12o
0
20
40
incident angle [o]
Magnetic field effect

Negative angle:
smaller cluster size

Positive angle:
bigger cluster size
track
track
n side
+ e- direction
y
Fm
x
+
+
Fm
+
Fe
Fm
++
B .
E
p side
Degradation of SVD intrinsic resolution ｄｕｅ
to higher detector occupancy (background):
cluster distortion or cluster mis-association?
residual
occupancy  0.3
Number of clusters with rel.
change in E > 0.2
Relative fraction of SVD hits
occupancy < 0.04
Number of
clusters with MC
hit (correctly
associated)
Number of
clusters with
MC hit
(correctlywrong
cluster
associated)
association
is negligible
occupancy, x coordinate
residual
Alignment cross-check
Check if residual distributions for individual
SVD units are shifted (mis-aligned)
 Try to correct
r coordinate
the shift

residual [cm]
Conclusions on the SVD intrinsic
resolution study

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Best resolution (RMS) at small track incident
angle is found to be 12 and 30 m for r and z
coordinate, respectively.
Results of this study provided important
information for

cross-check of the SVD alignment
 different geometry design to take into account
magnetic field effect
 improvement of clustering algorithm in the case of
higher detector occupancy
Analysis of data collected at the
Belle detector:
B0
fCP
B0
B0
Measurement of Time
Dependent CP Violation
in B0 → D+D- Decays
fCP
B0
+
DD
→
CP eigenstate
0
B
B0
c
d
tree
d
Vtd t
W-
b
•
•
•
Vtb
Vtb*
t
Vtd
D+
c
b
W+
*
D-
d
B0
d
c
B 0 and B 0 mixes with each other
penguin
c
via “box-diagram”.
The box-diagram includes CKM complex phase.
A path from B 0 to fCP via mixing has different weak phase from one
from B 0 to fCP directly due to the CKM phase.
Time-dependent decay amplitude
PCP±(t) = exp(-|t| / ) / (4) ·
(1±(1-2w)(S sin(mt)- C cos(mt)))
 S = - sin(2) (if only tree diagrams are present)

CKM triangle
Analysis
final step: fit S and C to t distribution
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Reconstruct the events
Measure the time of B meson decay from the
reconstructed B vertices.
Determine B0 flavor
Properly fit the t distribution (taking into
account detector resolution, mis-tagging of B
mesons,…)
e-: 8.0 GeV e
e+: 3.5 GeV
Brec
e+
fCP (fKS)
Y(4S)
~ 0.425
Flavor tag
z  c tB ~ 200 mm
Basc
z
5 steps toward the CP asymmetry measurement
•
•
•
•
•
Reconstruct B  fKS decays
Measure proper-time difference: t
Determine flavor of Basc
Evaluate asymmetry from the obtained t distributions
Discuss observed CP asymmetry
Event reconstruction
Br ~ 1.7 10-4
 D mesons are reconstructed only in decays to
charged particles (10%)
 Detector efficiency for reconstructing such an event
~ 10 – 20 % (at S/B ~ 1)
Expect only few (2-3) events per 10 M BB events!
High luminosity B-factory is needed – KEK!
Approx. 350 M recorded BB events – about 100 of
them will be correctly reconstructed as B0 → D+D
B0 decay time
Average distance between the two decay
vertices is 200 μm
 Need to measure the vertex position with
better accuracy
 With SVD we are able to measure the
vertex z coordinate with accuracy about
100 μm

Flavor tagging
t
t =tasc
Brec = B 0
Brec
(4S)
t =trec
B 0-B 0 mixing
???
D+D- decay
Basc
Brec = B 0 decay
(flavor-specific)
Use flavor-specific decays ( slow, K, leptons)
Current status
Event selection is optimised according to
best signal to background ratio.
 Vertex position and tagging are
determined using standard Belle software
 Fitting procedure is under development

(I can not show any preliminary results)
Conclusion…
It is great to
RICH
work for
SVD
Study of CP
violation